CVE-2026-107838 RIOT: nanocoap_fileserver ignores response initialization failure, leading to reachable assertion
Remote attackers can cause a denial of service in RIOT OS devices by triggering an assertion failure in the nanocoap fileserver when token extensions are enabled.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-107838?
CVE-2026-107838 is a high-severity denial of service vulnerability in RIOT-OS RIOT, published on 9 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
RIOT is an open-source microcontroller operating system designed for Internet of Things devices and other embedded systems. From version 2023.07 through version 2026.07, nanocoap_fileserver callers in sys/net/application_layer/nanocoap/fileserver.c ignore a failure returned by _resp_init() when coap_build_reply() cannot fit a response header into the response buffer. A remote client can send a CoAP request with a sufficiently large extended token when nanocoap_token_ext is enabled, causing response initialization to fail while _get_file() or _get_directory() continues with stale response state. The path then reaches _calc_szx2() and its pdu->payload_len > reserve assertion, terminating the affected service or device task. No fixed release is available as of this review.
Which versions are affected by CVE-2026-107838?
How do I fix CVE-2026-107838?
No fix has been published for CVE-2026-107838 yet. Limit exposure of the affected software until one is available.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-107838 being exploited?
There is no known exploitation of CVE-2026-107838 in the wild and no public exploit code.
Sources
Also known as GHSA-39j3-3v73-5mj2.